The present invention relates to the art of non-invasive anatomical examination. It finds particular application in conjunction with cardiac and respiratory gated magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the invention may have application in other non-invasive examination techniques in which the examination is controlled, gated, or modified in accordance with anatomical motion.
Magnetic resonance imaging sequences commonly include the application of a radio frequency pulse concurrently with a slice select pulse in order to excite magnetic resonance in a selected slice of the patient or subject. A phase encode gradient pulse is applied to encode phase into the resonating nuclei. Another radio frequency pulse is applied, either before or after the phase encode gradient pulse, to invert the magnetic resonance and cause a magnetic resonance echo. A read gradient pulse is applied during the echo to provide a second dimension of encoding in the retrieved magnetic resonance signal or view. Commonly, this sequence is repeated a multiplicity of times, each time with a different amplitude phase encode gradient in order to generate a corresponding multiplicity of differently phase encoded views.
Anatomical movement, such as cardiac and respiratory motion tend to degrade the resultant images. The amount of degradation is related to the amount or magnitude of physiological displacement from view to view, the rate of movement, and the like. Various anatomical condition monitors have been utilized to control the collection, processing, or use of magnetic resonance and other noninvasive imaging data in accordance with physiological motion. See for example U.S. Pat. Nos. 4,763,075 to Weigert and 4,545,384 to Kawachi.
A patient's cardiac cycle is normally sensed with electrocardiographic electrodes mounted to the patient's skin and connected by electrical leads with processing circuitry. In magnetic resonance imaging, the changing gradient magnetic fields induce an electrical response in the patient as well as in the electrocardiographic leads. This electrical response becomes superimposed on the electrocardiographic signal.
Conventionally, the magnetic resonance field gradients were applied as square pulses. A square pulse is the sum of sine waves--predominantly high frequency sine waves. The frequency content of the noise induced by square gradient pulses was significantly higher than the R-wave portion of the cardiac signal, the highest frequency portion of a normal cardiac signal. This difference in frequency enabled the R-wave and the gradient pulse induced noise to be separated, e.g. with a slew rate filter.
One of the problems with square gradient pulses is that they are relatively energy consumptive. It has been found that utilizing more rounded gradient pulses produces equally good images, but with significantly less energy. However, rounding the square wave pulses has reduced their frequency content. More significantly, the electrical response induced in the patient and leads by the rounded gradient pulses includes significant components that have a frequency in the same range as the frequency of the R-wave.
The present invention contemplates a new and improved anatomical gating system which overcomes the above referenced problems and others.